This invention relates in general to tube-type vessels and more particularly to a tube-type vessel having crevice-free joints at the ends of its tubes and to a method of manufacturing and assembling such vessels.
In the typical tube-type heat exchanger, tubes extend between tubesheets to which they are welded, thus creating two isolated fluid passages -- one through the tubes and another around the exterior surfaces of the tubes. These passages accommodate different fluids at different temperatures, thus enabling a transfer of heat through the walls of the tubes from the fluid of higher temperature to the fluid of lower temperature.
Various procedures exist for joining the tubes to the two tubesheets at the ends of the heat exchanger. One of the easiest is to merely extend the tubes loosely into bores within the tubesheets and then roll the tubes from within their interiors into tight contact with the surfaces of the bores, but this leaves the heat exchanger with crevices at the back faces of its tubesheets, where the tubes enter those tubesheets. Certain fluids when in the vapor phase tend to deposit precipitates in crevices, and these deposits, when in a crevice surrounding a tube, may girdle the tube, even to the extent of causing a total collapse. Other fluid becomes extremely corrosive in crevices. Aside from that, a tube that is simply expanded against its tubesheet does not create a positive connection with the tubesheet and is thus more susceptible to leakage than it would be if welded to the tubesheet.
Indeed, it is common practice to weld the tubes of a heat exchanger to the tubesheets at its ends, and this is most conveniently achieved by extending the tubes completely through their respective bores in the tubesheets and welding their ends to the front faces of the tubesheets, that is to the faces which are presented away from the fluid that is circulated through the spaces between the tubes. But this procedure, even though it produces a positive connection between the tubes and the tubesheets, still leaves crevices at the back faces of the tubesheets where the tubes enter the bores in such tubesheets. The fluid that is in the spaces between the tubes may enter these crevices and leave deposits or may effect a corrosion of the tube and tubesheets.
A process exists for eliminating the crevices at one of the tubesheets, and this process involves forming a short counterbore or socket at the front face of a tubesheet where the bore for each tube opens out of the front face. The tube for the bore fits into the socket, and is welded to the tubesheet autogenously from within the bore. This leaves a crevice-free weld around the tube where it enters the tubesheet. In this procedure the end of the tube projects a specified distance into the tubesheet, in order to achieve an effective weld. Because the tubesheets are large and very heavy, it is impossible to position them perfectly parallel for welding. As a consequence, the tubes are secured to only one of the tubesheets using this procedure, and at the other tubesheet they are brought completely through their respective bores and welded to the front face of that tubesheet. This leaves the heat exchanger with crevices in the back face of only one of its tubesheets, but this is often not a problem, because the heat exchanger can be positioned such that the fluid which is against the creviced back face is less likely to produce crevice buildup or corrosion.
The process for producing fillet-type welds at the back face of one of the tubesheets is disclosed in U. S. Pat. Nos. 4,221,263 and 4,535,214.
In other heat exchangers, it is not enough to have crevice-free joints at only one tubesheet; they need to be at the opposing back faces of both tubesheets.